The invention relates to optimizing a process to remove normal paraffins from a feedstream. In particular, the present invention relates to optimizing a process based upon the determination of the normal paraffin content of kerosene.
Petroleum distillates in the kerosene boiling range may contain up to 30 weight percent (wt %) normal paraffins. In order to meet product quality specifications for refined kerosene, the normal paraffin content must be reduced to below 5 wt %. The normal paraffins which are removed are fractionated and sold as additional products.
Normal paraffins may be removed from kerosene by selective absorbers, such as certain zeolite molecular sieves, or membranes. In a zeolite separation process, pairs of fixed beds containing these zeolites are used in commercial processes, to continuously remove the normal paraffins from kerosene feeds. In this process, for example, the feed is vaporized and introduced into one of the beds. As the kerosene vapor front propagates through the bed, normal paraffins are selectively retained in the pore structure of the molecular sieves. The material which emerges from the bed, often called the "sievate", is depleted in normal paraffins. As used herein, sievate means the product of a separator which includes all of the feed components, other than the separated normal paraffins. The material retained in the bed, called the "desorbate", is comprised almost entirely of the normal paraffins which have been removed from the kerosene.
When the bed becomes nearly saturated with normal paraffins, the feed is switched to the second bed. While the second bed absorbs the normal paraffins from the kerosene, the paraffins that are retained in the first bed are desorbed by ammonia vapor, as an example, and subsequently recovered for further processing. The cycle time is approximately six minutes in commercial-scale processes.
Variation in the normal paraffin content of kerosene feedstocks, the lack of an infinitely sharp kerosene vapor front, and the lack of complete absorption selectivity by the molecular sieves are factors which contribute to a variation in the optimum time at which the feed should be switched between beds.
There are economic benefits to switching the feed between beds at the optimum time in the absorption process. Reduced through-put and higher energy costs for inefficient desorption result if the switching time is shorter than optimum. Contamination of the sievate stream by normal paraffins occurs, if the switching time exceeds the optimum. Product quality is reduced and excessive costs may result from storage of off-spec product and, possibly, from reprocessing.
The optimum switching time may be determined in two ways. Measurement of the normal paraffin content of the feed can be used with an accurate process model to compute the optimum time for the particular feed and process conditions. Alternatively, the normal paraffin build-up in the sievate can be monitored to determine the time to switch beds before contamination of the sievate occurs. Both control strategies require a rapid, accurate, and precise on-line determination of the normal paraffin content of the kerosene feed or sievate.
Gas chromatography (GC) is the method currently used to determine the normal paraffin content of kerosene. The method has poor reproducibility due to thermal decomposition of the paraffins on the chromatographic column upon thermal desorption. The repeatability of the method is approximately 3 wt %. In addition, the measurement time is about 20 minutes and,therefore, exceeds the 6 minute process cycle time. The present invention is based upon absorption of light which overcomes the limitations of the GC technique.